by Eric Greene

Iceland spar is a clear, transparent, colorless crystallized variety of calcite (calcium carbonate, CaCO3). Large pieces are split along natural cleavage planes to form natural rhombs. Iceland spar is probably best known for exhibiting the optical property of double refraction - so, anything viewed through the crystal appears double. It has many uses, in everything from precision optical instruments to LCD screens, and was even used during WWII to make bombsights. The perfect, flawless, colorless Iceland spar that is used in optical instruments sells for more than $1000 per kg. Historically, the first, best quality, and most abundant source of this clear calcite was in Iceland, which is where it got its name. Recently, Iceland spar has been in the news because of new research that shows that this mineral was almost certainly used by the Vikings for navigation.
The scientific term for double refraction is bifringence. This phenomenon occurs when a ray of light enters Iceland spar, and is split into two beams of polarized light, creating two images. Each image corresponds to one of the rays of light which has been split off and polarized. The light rays travel at different speeds, and are bent into two different angles (known as angles of refraction). When you look through Iceland spar, you see two images of everything. A good way to view double refraction is to put a piece of Iceland spar on top of a piece of paper with a single straight line; looking inside the piece, you will see two lines. The thickness and angle of rotation of the Iceland spar determines the distance between the lines. If you rotate the piece to the right angle, the two lines will merge into a single line.
Polarized light occurs when light waves vibrate in a single plane. This property is responsible for many natural phenomena, including blue skies, rainbows, bird migration, and more. Light consists of electromagnetic waves with both electric and magnetic fields. If these components have a specific orientation, the light is said to be polarized; in un-polarised light the orientation of these fields has no preferred direction.
In mineralogy, cleavage refers to the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the consistent locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye. Most people are familiar with the one-directional cleavage of muscovite mica. The two-directional cleavage of calcite is so flawless that if a crystal is struck with a hammer, it will usually break into perfect rhombohedra.
Over 1,000 years ago, the Vikings sailed from Scandinavia to America without the use of magnetic compasses, astrolabes, maps or any other known device. Yet they still managed to find their way in spite of the clouds, fog, and long summer twilights characteristic of near Polar Regions. Centuries old Viking legends attribute their navigational success to the use of mysterious, glowing "sunstones" to find the position of the sun and set the ship's course even on cloudy days. Although still controversial, many researchers now believe that these "magical sunstones" were in fact Iceland spar. 
When Iceland spar splits light into two polarized rays, the two different images will have different brightness depending on the polarization. Because sunlight is polarized when it enters the Earth's atmosphere, it is possible to change the orientation of a piece of Iceland spar to determine the direction of the sun. This is done by moving the crystal until the relative brightness of the two images are equal, which only happens when the crystal is aligned to the sun. This is possible even when the Sun is hiding behind clouds or just below the horizon.
However, it can be very difficult to determine when the two images are of equal brightness. A recent study suggests that the Vikings may have built a simple device to better use the sunstone. The crystal is covered with a solid screen with a small hole in its centre, and a pointer. When light passes through the hole onto the crystal, a dark surface below it receives the projection of the double image for comparison. Experiments showed that by rotating the device until the two images were equally bright, the sun's position on a cloudy day could be determined to within 1º of accuracy, and that it is also very accurate even when the sun is below the horizon.

Although archaeological excavations of Viking villages have not turned up any Iceland spar, the mineral was found in the wreck of the 16th century ship, Aldemey Elizabethan, where it was probably used for navigation (note: the presence of iron cannons on a ship can throw off a magnetic compass by up to 90º).
The transparent crystal formation of calcite is called Iceland spar because it was first found in Iceland in mines that extracted the mineral from large cavities in frozen lava. Here are the physical characteristics of Iceland spar: 
Streak: White Hardness: 3            Crystal System: Hexagonal          Transparency: Transparent 
Specific Gravity: 2.7                        Lustre: Vitreous                                Cleavage: 1,3 - rhombohedral 
Fracture: Conchoidal (rarely noticeable due to the perfect cleavage) 
Tenacity: Brittle
In Europe in 1669, the Danish scientist Rasmus Bartholin (1625-1698) discovered and studied double refraction in calcite. Other early scientists who studied this crystal included Christiaan Huygens (1629-1695), Sir Isaac Newton (1643-1727), and Sir George Stokes (1819-1903). Originally discovered in and named after Eskifjord, Iceland, where in the 19th century giant crystals of calcite up to 25 feet were found. High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry. The Norden bombsight patented in 1941 utilized calcite in the lenses to gauge bomb delivery based on the speed and altitude of the plane compared to its target.
Iceland spar has been found in many localities around the world. In addition to the long-closed Helgustadir Mine, in Eskifjord, Iceland, important producers today include Santa Eulalia, Chihuahua, Mexico; the Iceberg Claim at the Harding Mine, in Dixon, New Mexico; the Cabacal Mine in Mato Grosso, Brazil; and more recently, from China. Coloured Iceland spar is found in various shades of yellow, green, pink and more.

The historic Helgustadir mine is on the northern shore of the Reydarfjord on the outer east coast of Iceland. This deposit Discovered in the 17th century, it was mined on and off until 1924. The crystals were transparent enough for use by the optical industry, and specimens were also distributed among the leading European scientists of the time. The concepts of double refraction and crystal cleavage planes were first discovered on calcite crystals from this site, and helped mineralogists gain a better understanding of crystal physics. Now the mine is covered with snow for most of the year, and is so remote that it is nearly inaccessible. Iceland declared Helgustadir a national heritage site in 1975, so today collecting is forbidden.
We have printed the excellent article above with the permission of Eric Greene of Treasure Mountain Mining. Optical calcite was also mined in the past at Soetwater, near Calvinia.
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